Hopkinson peak and superparamagnetic effects in BaFe12-xGaxO19 nanoparticles

The European Physical Journal Conferences 01/2012; 29(00039):1-8. DOI: 10.1051/epjconf/20122900039


In this article, the thermomagnetic properties of a system of
Ga-substituted barium hexaferrite nanoparticles
(BaFe12-xGaxO19) prepared by ball
milling were investigated. The thermomagnetic curves for the samples
with x ranging from 0.0 to 1.0 exhibited sharp peaks with high
magnetization just below TC (Hopkinson peaks). The height of
the peak for our samples was similar or larger than previously observed
or calculated values. Theoretical treatment of the experimental data
demonstrated that the peaks are due to the effect of superparamagnetic
relaxations of the magnetic particle. This effect was confirmed by
hysteresis measurements at, and just below the temperature at which the
peak occurred. Consequently, the particle diameters were calculated from
the experimental data using a theoretical model based on the
superparamagnetic behavior of a system of uniaxial, randomly oriented,
single domain, non-interacting particles. The calculated diameters of 11
- 26 nm are less than the physical diameters determined from TEM
measurements. The factors responsible for the low calculated values are

Download full-text


Available from: Sami Mahmood
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Synthesis of barium hexaferrites BaFe12O19 (BaM) is often accompanied by the presence of secondary nonmagnetic phases. The coexistence of these phases reduces the yield of the desired BaM magnetic phase and screens its intrinsic magnetic properties such as the saturation magnetization, and impacts the magnetic properties of the sample negatively. Therefore, assessment of the abundance of these phases and investigating their effect on the structural properties of the sample is of fundamental and practical importance. In this work, BaM hexaferrites were prepared by ball milling and sintering powder precursors with Fe:Ba molar ratios varying from 11.5 to 16.16. The structural properties of the phases in the samples were investigated by x-ray diffraction (XRD). The weight ratios of the different phases, as well as their refined structural parameters were determined using Rietveld analysis. XRD patterns revealed the development of α-Fe2O3 (hematite) phase with increasing relative diffracted intensity as the Fe:Ba molar ratio increased. The evolution of the intensity of this phase was used to monitor the weight ratio of the secondary hematite phase in the sample, and a relation between the its weight ratio and the Fe:Ba ratio was established. The optimal Fe:Ba ratio required to synthesis a pure barium hexaferrite phase was then determined, and found to be 11.7.
    Full-text · Article · Oct 2015 · IOP Conference Series Materials Science and Engineering
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We carried out a systematic study on the effect of the substitution of Ti2+ and Ru4+ ions for Fe3+ ions on the structural and magnetic properties of the strontium ferrite SrFe12-2xRuxTixO19 nanoparticles with (0 ≤ x ≤ 0.3), using x-ray diffraction, Quantum Design PPMS-9 magnetometry, and electrical resistivity. A clear irreversibility between the zero-field-cooled and field-cooled curves was observed below room temperature and the zero-field-cooled magnetization curves displayed a broad peak at a temperature TM. These results were discussed within the framework of random particle assembly model and associated with the magnetic domain wall motion. The resistivity data showed some kind of a transition from insulator to perfect insulator around TM. The high-temperature magnetization measurements exhibited sharp peaks just below Tc indicating a superparamagnetic behavior. With Ru-Ti substitution, the saturation magnetization at 5 K showed small variations were it slightly increased with increasing x up to 0.2, and then decrease for x = 0.3, while the coercivity decreased monotonically, recording a reduction of about 78% at x = 0.3. These results were discussed in light of the cationic distributions based on the results of the structural refinements.
    Full-text · Article · Jul 2015 · Journal of Alloys and Compounds
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: BaFe12-xCuxO19 hexaferrites were prepared using ball milling and sintering at 1100 °C. Refinement of the x-ray diffraction patterns was carried out to determine the structural parameters and the ionic distribution over the crystallographic sites. The preferential site occupation and valence state of Cu was consistent with the results obtained from the analysis of Mössbauer spectra. Further, the magnetic parameters of the samples were discussed in light of the structural and Mössbauer analyses. The magnetic phase transition temperature was found to decrease with the level of Cu substitution, in accordance with the reduction of the superexchange interactions. Further, the magnetic softening of the hexaferrite and the significant reduction in magnetocrystalline anisotropy with Cu substitution was consistent with the ionic distribution in the lattice. This study clearly demonstrated the feasibility of using a simple method to fabricate hexaferrites with a modified coercivity, while maintain the saturation magnetization high enough for practical applications. Highlights • Single BaM hexaferrite structural phases with Cu substations were prepared. • The magnetocrystalline anisotropy decreased with Cu substitution. • The coercivity was significantly modified while the magnetization remained high. • Hexaferrites with 0.2–0.4Cu possess properties suitable for magnetic recording. • Ionic distributions from structural refinement agreed with Mössbauer spectroscopy.
    Full-text · Article · Oct 2015 · Materials Research Bulletin
Show more